Thermoplastic polyethylene is commercially produced by three important classes of catalysts, namely free radical catalysts (such as the peroxides and/or hydroperoxides which are typically used in the so-called "high pressure" polymerization process); chromium catalysts (such as the supported chromium oxides which are used in the so-called "Phillips" polymerization process); and "Ziegler-Natta" type catalysts which are typically used in "gas phase" processes and "medium pressure solution processes".
The polymer solution emerging from the reactor system in the medium pressure polymerization process still contains unreacted monomers and active catalyst which would continue an uncontrolled polymerization reaction in the process equipment down-stream from the polymerization reactor system and thus compromise the quality of the desired commercial polymer. Therefore the catalyst has to be deactivated.
There are many deactivators known including various amines (see, for example, U.S. Pat. No. 4,803,259 to Zboril et al; alkali or alkaline earth metal salts of carboxylic acid (especially calcium stearate, per U.S. Pat. No. 4,105,609 to Machon et al); water (U.S. Pat. No. 4,731,438 to Bernier et al); and hydrotalcites (or synthetic clays) as disclosed in U.S. Pat. No. 4,379,882. In fact most polar compounds will deactivate a Ziegler catalyst at the typical temperature at the reactor exit.
However, most Ziegler-Natta catalysts contain halogens (typically chlorine) which remain in the polyethylene and may cause undesirable reactions (especially corrosion of metals which come into subsequent contact with the polyethylene). In a solution polymerization process, these undesirable reactions may occur in process vessels which are immediately downstream of the polymerization reactor so there is a need to employ an effective "deactivator" either in, or downstream from, the polymerization reactor.
Preferred deactivators should also satisfy the following requirements: a deactivator must deactivate the catalyst rapidly; must not deposit on the equipment (particularly on heater surfaces); must not generate color or odor and must be safe and non-toxic. This limits the types of useful deactivators and dictates the way they are added to the reactor effluent. Accordingly, the selection of optimal deactivators and the method of their use depends upon the process in question.
The method of adding the deactivator is affected by the form of the polyethylene product and the type of polymerization reactor. In general, it is not particularly difficult to add a deactivator to the solid product from a gas phase or slurry polymerization process. (See, for example, the aforementioned U.S. Pat. No. 4,731,438 which discloses that water may be added to the solid product from a gas phase polymerization process by simply spraying the water into a purge bin.) Likewise, it is not particularly difficult to deactivate the molten solution product which emerges from a high pressure, plug flow tubular reactor--as the deactivator may be added directly to the end of the reactor tube. (See, for example, U.S. Pat. No. 4,634,744; Huang et al). However, the direct addition of a deactivator at the exit of a back-stirred reactor (such as a CSTR) would kill the reaction.
Conversely, the addition of a particulate deactivator to the polyethylene solution at a point downstream of the reactor is not trivial--particularly with respect to the problem of achieving fast deactivation by adequate dispersion of the deactivation throughout the solution.
Often, it is advantageous to separate the catalyst deactivation and passivation. Thus a soluble deactivator such as methanol may be added first, and a suspension of a passivator second. Methanol mixes well and deactivates quickly, but the so-deactivated catalyst must also be passivated.